1. Field of the Invention
The present invention pertains to systems and methods for producing adjustable beams, spot sizes and spot shapes on a target.
2. Background Information
Today, many ophthalmic treatments involve using an optical beam to treat a target (e.g., a patient's eye). For example, diabetic retinopathy and age-related macular degeneration are subject to photocoagulative treatment with visible laser light. In performing these treatments, it is sometimes advantageous to be able to use beams of different sizes depending on the particular type of treatment and the condition of the patient. Traditionally, adjustable optical beam diameters have been produced by using a fixed light source with either a zoom lens or a turret assembly to vary the magnification level. Alternatively, the optical beam has been defocused by changing the distance between the target and the last lens in the chain of optical elements to vary the beam spot size. While these methods vary the beam spot size satisfactorily, these methods involve moving elements with large moments of inertia. Having to move elements makes the system expensive to build and operate and, more importantly, limits the speed at which the beam spots can be adjusted.
A system using a zoom lens has additional problems stemming from the second law of thermodynamics which, in the context of optics, makes the source the brightest part of the system. A system's optical invariant is represented by the following equation:Invariant(I)=ypnu−ynup                 where yp and up are the height and slope angle of the principle ray and                    y and u are the height and slope angle of the marginal ray and            n is the index of refraction                        
At the object and conjugate image planes, the Invariant reduces to:                Object plane: I=honouo Image Plane: I=hiniui                     where h is the height of the object/image and the subscripts o and i denote the object and image locations.                        
In the context of this application, the paraxial approximation dominates and the product nu can be replaced with NA, numerical aperture. Therefore, the optical invariant can be written:I=ho(NA)o=hi(NA)i 
Magnification of the optical system is:M=hi/ho=(NA)o/(NA)i 
wherein M=magnification level,                (NA)0=numerical aperture on the object side, and        (NA)i=numerical aperture on the image side of the system.        
If a single object is used, the image side numerical aperture will therefore decrease with increasing image spot size. Because the use of larger spot sizes requires proportionally higher power to cause the desired effect on the target (e.g., drilling a hole or heating tissues), this inverse relationship between large spot size and small numerical aperture poses a problem when there is an intermediate structure between the source and target. For example, these intermediate structures can often absorb enough of the delivered light to cause damage to itself and the system. This may occur, for example, in the case of transpupilary retinal photocoagulation.
There is a need for systems to have the ability to vary spot size at the treatment plane. The spot size variation allows for flexible adaptation of treatment fluences and geometries. Furthermore it is also practical to have the ability to easily switch the treatment beam on and off without disrupting the stability of the light source for instance. It is further advantageous to have the ability to scan a pattern of treatment light on the targeted structure. This scanning ability overcomes the tedium and duration of treatment when single spots are laid down one at a time. The present invention meets the needs of; varying spot size; switching the treatment light off/on; and scanning and does so in an efficient manner using a well balanced distribution of the attending functions in an economical and compact package.
In the described embodiments, the switching mechanism is achieved via a galvanometric (galvo) moving mirror technique at the input to the fiber. The galvo mirror technique is economical while achieving appropriate on/off switching times, however other means of optically deviating the beam are also considered.
Scanning in the treatment area is also achieved using galvanometric moving mirrors using a separate set of mirrors on the output side of the fiber. Once again, the choice of a galvo technique is economical and compact while achieving adequate scan range, resolution, and speed.
Adjustment of the spot size at the treatment plane has been achieved traditionally in several different ways. One simple technique is to defocus the beam at the treatment plane. The defocus technique has the disadvantage of deviating from the image plane and therefore an uncertain change in irradiance distribution. There is also the loss of the sharp edge definition of the spot and the safety issue of the positioning a small beam at some location other than the treatment plane. Another method to vary spot size is to employ a variable magnification optical system. This can be done by inserting and replacing sections of the optical systems as in a turret design. A zoom lens configuration whereby axial distances between lenses are adjusted can also be used. Both the turret and zoom lens configurations involve movement of powered optics and therefore the associate disadvantages in reliability and alignment. Also the lens systems are complex and the optical performance is a weighted compromise over all possible configurations. This is particularly true for a zoom lens design.
Moreover, traditional approaches relate only to limited aspects of the overall etendue transfer characteristics of interest, such as magnification or aperture size change in one optical element as described immediately above. Consideration of the overall etendue, or changes in etendue as a function of one or both of fiber-related parameters or variations resulting from optical elements that modify an effective etendue of a beam, is based on the relationship set forth by the basic expression of this invariant (luminosity, throughput, or etendue), or “G,” represented by the following equation:G=n2AΩ                wherein A is the area of the entrance pupil of the optical element,                    Ω is the solid angle subtended at this pupil by the ray, and            n is the refractive index of the media between the two.                        
The optical invariant, I, is the reduction of the etendue to its linear or one dimensional form and may be more commonly known.
This relationship should also be considered in connection with those pertaining to magnification and numerical aperture, as set forth above. Therefore, drawbacks exist for system that are unable to change beam diameter, spot size and spot shape via changes to one or more of fiber diameter (core size), effective fiber diameter (and/or effective numerical aperture), aperture size, and entendue-modifying aspects such as aperture placement, among others that may be varied vis-á-vis application of the above-stated relationships to elements or articles of optical systems.
Accordingly, apparatus and methods for providing adjustable beams, spot sizes, and spot shapes without the above-described limitations and disadvantages are desired.